(1) Field of the Invention
The present invention relates to a tractor power take-off system in which the PTO shaft is projected from the rear end of the transmission case having therein a PTO gear speed-change mechanism for changing the number of rotations of the PTO shaft.
(2) Prior Art of the Invention
A conventional tractor has been constructed such that a working machine is attached to the rear end thereof and is driven, through the PTO gear speed-change mechanism, by the PTO (power take-off) shaft projected from the rear end of the transmission case.
There have recently been proposed improvements in such a conventional tractor in which another PTO shaft is disposed so as to drive a so-called "mid-mount" working machine, for example a mower, attached to the center position of a tractor.
However, it disadvantageously increases the manufacturing cost to manufacture, in different processes, a transmission case for a tractor to which a mid-mount working machine above-mentioned is attached, and a transmission case for a tractor to which such mid-mount working machine is not attached. It is rather desired to use common parts as much as possible for either type tractor.
On the other hand, when driving a mid-mount working machine above-mentioned, such working machine is required to be driven with its speed changed according to its machine kind or working conditions. Provision of another PTO gear speed-change mechanism required for such speed-change will further increase the manufacturing cost of a tractor.
As one of the measures to overcome such a defect as disposing another PTO gear speed-change mechanism, it may be proposed to commonly use one PTO gear speed-change mechanism for both the first and second PTO shafts.
According to such structure, when performing a speed-change operation, the speed-change gear of the PTO gear speed-change mechanism should be shifted to simultaneously engage with a gear at the driving shaft side and a gear of the first or second PTO shaft. However, such engagement may not be performed in a satisfactory manner.
In view of the foregoing, there has been desired a tractor power take-off system in which a speed-changeable first PTO shaft and a speed-changeable second PTO shaft may be disposed without considerable modification of a conventional transmission structure, and speed-change operation of such first and second PTO shafts may smoothly be performed.